Interferometry is one of the most sensitive optical interrogation methods and has been used in a wide array of technologies including astronomy, fiber optics, engineering metrology, quantum mechanics, plasma physics, remote sensing, and biomolecular interactions such as screening molecular interactions in surface binding modes. Several types of interferometry have been developed such as fluorescence interferometry for high resolution microscopy or nanoscopy, label-free sensing based on a Mach-Zehnder Interferometer, a Young Interferometer, a dual polarization interferometer, back-scattering interferometry, and spectral reflectance interferometry, to name a few.
Surface Plasmons (“SPs”) are coherent oscillations of conduction electrons on a metal surface excited by electromagnetic radiation at a metal-dielectric interface. The sensitivity of the Surface Plasmon Resonance (“SPR”) to the refractive index change at a flat metal interface has led to the development of SPR sensing systems based on interferometry and that use prisms to couple light into a single surface-plasmon mode on a flat, continuous metal film (e.g., gold). However, the relatively large size of these experimental systems is a disadvantage for applications requiring integrated, low-cost, compact, image-based devices for portable, rapid bio-analytical measurements.
Nanoplasmonic biosensors, employing nanoscale metal particles, provide an attractive miniaturized platform for sensitive, label-free monitoring of cellular processes. When receptor molecules are immobilized on the nanostructured metal surface, the binding of target biomolecules changes the local refractive index, which affects the optical properties of the SP modes and permits optical detection. Recent advances in nanofabrication, nanomaterial synthesis, and nanocharacterization permit significant advances over conventional SPR evanescent wave-based biosensors, whose large size limits their effectiveness for probing nanovolumes and single cells, and for integration into microfluidic platforms. However, the sensitivities for these nanoplasmonic structures are much lower (two to three orders of magnitude) than other sensitive optical sensing technologies.